1. Field of the Invention
The present invention relates to direct conversion receivers and circuits and methods for receiving and mixing radio frequencies, and more particularly to a circuit and a method that can diminish a secondary-order inter modulation distortion at a direct conversion receiver.
2. Description of the Related Art
Generally, the direct conversion receiver or a homodyne receiver provides advantages compared to a superheterodyne receiver.
FIG. 1 is a radio frequency receiving circuit diagram of a conventional direct conversion receiver (DCR).
Referring to FIG. 1, the direct conversion receiver transforms an input signal into an inphase signal and a quadrature signal having a baseband frequency without transforming the input signal into a signal having an intermediate frequency (IF).
A radio frequency (RF) signal received by an antenna 10 is inputted to a low noise amplifier 12, and then an output signal of the low noise amplifier 12 is inputted to each of a first mixer 14 and a second mixer 16.
At the first mixer 14, the amplified radio frequency signal is mixed with a local oscillator signal 20 such as a cosine wave signal of a local oscillator 20. The local oscillator signal 20 has a same frequency as a carrier frequency. At the second mixer 16, the radio frequency signal is mixed with a sine wave. The sine wave has phase difference of 90° with respect to the local oscillator signal 20, and generated by a π/2 phase shifter 18.
The first and second mixers 14 and 16 generate the inphase signal and the quadrature signal, respectively, which have a mean frequency such as the baseband frequency and a harmonic frequency such as a twice carrier frequency (2fc). Harmonics of the signals generated by the first and second mixers 14 and 16 are removed by two low pass filters 22 and 24, respectively. The inphase signal and the quadrature signal having baseband frequency are amplified and outputted by two amplifiers 26 and 28, respectively.
The DCR has a simple circuit configuration compared with the superheterodyne receiver, and is easier to implement as an integrated circuit. The a minimized DCR circuit can be manufactured at a low cost.
However, the DCR has some problems. One of the problems is a secondary intermodulation distortion generated by the mixer. The secondary intermodulation distortion is caused by the mixer having a nonlinear active device. A harmonic frequency component of the output signal is generated by a radio frequency signal process using the nonlinear active device, and may be a sum or difference of the harmonics of two different input signals. A DC offset is generated in addition to unwanted secondary harmonics by a non-linearity of the mixer.
When two input signals respectively having two frequency components f1 and f2 are inputted into a nonlinear circuit, frequency components such as 2f1, 2f2, f1+f2, 3f1, 3f2, 2f1−f2, 2f2−f1, 2f1+f2 or 2f2+f1 are generated due to the non-linearity of the nonlinear circuit as well as f1, f2. In general, a filter removes the frequency components caused by a non-linearity.
When the input signal frequencies f1 and f2 are slightly different from each other and an application defines the baseband frequency as the mean frequency, the frequency component of f1−f2 that is close to the baseband frequency is not removed by the filter. The frequency component due to the non-linearity is presented in the form of interference among channels having a small frequency difference, or in signal distortions by mutual interference of the signals in a signal band.
The frequency component of f1−f2 is referred to as the secondary intermodulation distortion (IMD2). The linearity of circuit is represented by a relation between the IMD2 quantity and a quantity of an amplified input signal frequency. A value representing the linearity of circuit is referred to as a second order intercept point (IP2).
Additionally, because the DCR shifts the frequency of the desired signal to the baseband, the IMD2 generated by the mixer can deteriorate the function of the DCR.
To solve above mentioned problem, some attempts have been suggested.
One of the attempts is to control mismatches of load resistances to equalize phases of the outputted secondary harmonics, and to equalize amplitudes of the outputted secondary harmonics, so that the secondary harmonics is removed by differential inputs. The effectiveness of the method of the matching load resistances depends on how finely the load resistances are controlled. However, the precise control of the load resistances is limited by a fabrication process of the integrated semiconductor circuit.
Another method is disclosed in Korean Patent Laid-Open Publication Nos. 2001-34820 (that corresponds to U.S. patent application Ser. No. 09/064,930), and 2002-68128.
In Korean Patent Laid-Open Publication No. 2001-34820, the IMD2 is transformed out of a pass band of a low pass filter and removed, by a switching operation of an inverter for an outputted signal polarity of a mixer,. In addition, a switching frequency of the inverter is high as compared with bandwidth of an input signal.
According to the disclosure in Korean Patent Laid-Open Publication No. 2002-68128, the IMD2 is minimized by circuit configuration biased in region in which a first differential function of transconductance of a complementary active device has a maximum and minimum values.